refactor(stammbaum): extract RelationshipChip and AddRelationshipForm

Split StammbaumCard from 366 to 196 lines by extracting:
- RelationshipChip.svelte — single relationship list item with optional delete
- AddRelationshipForm.svelte — self-contained add-relationship form with open/close state

Both components have browser-mode spec tests covering rendering and interaction.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/RelationshipChip.svelte.spec.ts

@@ -0,0 +1,48 @@
+import { describe, it, expect, afterEach, vi } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import RelationshipChip from './RelationshipChip.svelte';
+
+vi.mock('$app/forms', () => ({ enhance: () => () => {} }));
+
+afterEach(cleanup);
+
+const baseProps = {
+	chipLabel: 'Elternteil',
+	otherName: 'Anna Schmidt',
+	yearRange: '',
+	canWrite: false,
+	relId: 'rel-1'
+};
+
+describe('RelationshipChip', () => {
+	it('renders the chip label', async () => {
+		render(RelationshipChip, baseProps);
+		await expect.element(page.getByText('Elternteil')).toBeInTheDocument();
+	});
+
+	it('renders the other person name', async () => {
+		render(RelationshipChip, baseProps);
+		await expect.element(page.getByText('Anna Schmidt')).toBeInTheDocument();
+	});
+
+	it('shows year range when provided', async () => {
+		render(RelationshipChip, { ...baseProps, yearRange: '1920–1980' });
+		await expect.element(page.getByText('1920–1980')).toBeInTheDocument();
+	});
+
+	it('does not show year range span when empty', async () => {
+		render(RelationshipChip, { ...baseProps, yearRange: '' });
+		expect(document.querySelector('[data-testid="year-range"]')).toBeNull();
+	});
+
+	it('shows delete button when canWrite is true', async () => {
+		render(RelationshipChip, { ...baseProps, canWrite: true });
+		await expect.element(page.getByRole('button')).toBeInTheDocument();
+	});
+
+	it('hides delete button when canWrite is false', async () => {
+		render(RelationshipChip, { ...baseProps, canWrite: false });
+		expect(document.querySelector('button')).toBeNull();
+	});
+});